Chern. Anal. (Warsaw), 44, 1 (1999)

AluDliniuIDSpeciation in Natural Waters

by Krystyna Pyrzynskal,Ewa Bulskal,SerefGu~er2and Adam Hulanicki1

IDepartment(~fChemistry, University ofWarsaw,. Pasteura 1,02-093 Warsaw, Poland2Uludag University, Faculty ofScience and Arts, Department ofChemistry, 16058 Bursa, Turkey

Key words: aluminium speciation analysis, analytical methods, reviews

Thepresence<ofaluminiumin the form of different species in natural waters receivesan increasing attention due to better understanding of its bioavailability, toxicity andtransport mechanism.. Recently a number of nlethods for fractionating of alulniniumhave been developed. Most of them are· operationally defined, since the validation ofwhat is really measured· in real smnples is very difficult. This review concerns thespecific problems of aluminium speciation analysis. and highlights some importantmethods used for this purpose.

Badania dotyczqce toksycznosci, biodostvpnosci i obiegu gUnu w srodowisku natural­nym Sq zwiqzane z wystvpowaniem tego metalu w postaci r6znych zwiqzk6w w wodachnaturalnych. W ostatnich latach w literaturze ukazalo sivwieleprac opisujqcych r6znemetody badania specjacji glinu. Celemniniejszej pracyjest przedstawienie najwazniej­szych aspekt6w zwiqzanychzbadaniem specjacji glinu oraz krytyczne przedstawienieopisanych w literaturze metod analitycznych.

The chemistry of aluminium and itsspeciation becomes an area of intensive studyin recent years [1,2]. Due to limited solubility in the absence of complexing ligandsatthe pH normally encountered in most natural waters,alurninium content is low;the average concentration or10 Jlg 1-1 for the hydrosphere [3,4]·and of 100-500 flg 1-1for fresh water [5-7] has been reported. High concentration such as 1.6 mg 1-1 of Alin surface water from the Lysina catechumens in the Czech Republic has also beendetermined [8]. The present great interest in the· aquatic chemistry· of aluminiumoriginates, to a large extent, froin input of acids into the envirpnrnent, mainly throughfossil fuels containing sulfur or nitrogen. This leads to a decrease of pH in watersand soils with poorly buffered bedrocks, and cause a greater mobility of aluminiumin the environment. Aluminium solubilised by ,acids is toxic to plants because itantagonises calcium'binding essential for root cell surfaces [9]. The presence of somelabile aluminium species in surface waters is also toxic to fish [10].

2 K. Pyrzynska et al.

In clinical chemistry this metal is well known for its toxicity to uraemia patients[11]. Aluminium transfer through the dialysis membrane to the blood and its accu­mulation in bone and brain tis'sue have been associated with a series of clinicaldisorders suffered by the patients undergoing regular haemodialysis [12]. The etio­logic role of aluminium in Alzheimers disease and other neurodegenerative diseasesis still controversial [13].

The role of aluminium in complex ecosystems or living organisms, its tox­icity, reacti vity or bioavailability dependsmainly on its chemical form. Thereforethe chemical bonds in which it is involved and eventual association with othercomponents of given sample matrix are of a great importance. In aqueous solu­tions aluminium exists in a number of complexes, all of which are in dynamicequilibrium. Each species can also undergo several changes in a respect chain:A13+ +-+ AIOH2+~A12(OH)~+~ AI3(OH)~+~ AI8(OH)~O AI1304(OH)~t~

AOH)3' then, with an increase in· pH, the anionic fonns can appear [14]. Beside theimportance of pH, the distribution of aluminium species is depending also on the total metalconcentration, temperature and the presence of different inorganic and organic .ligands[15,16]. The A13+, AIOH2+ and Al(OR)! species are more reactive and toxic thanpolymeric forms and organically bound alumin,ium [17]. The main inorganic ligand,apart from hydroxide is fluoride, which forms very strong complexes with A13+. Sincefluoride concentration in natural waters is generally lower than that of aluminium,mainly Alp2+ and AIF2+ are present. The presence of fluoride ions can reduce thetoxicity of Al to fish and, as it was reported [18], reduces the incidence of dialysisdementia syndrome. It was found that aluminium sulfato, phosphato and silicatecomplexes also exist in natural waters. Recent studies have shown that the toxic effectof Al in natural waters is diminished in the presence of silica acid, as insolublehydroxoaluminosilicates are formed [19,20]. Most organic compounds with hydro­xylic and carboxylic groups could form stable complexes with aluminium. Theorganic ligands may be mainly humic and fulvic acids, since these constitute.a majorpart of the dissolved organic material in natural waters [21,22].

In the recent years various analytical methods for speciation of aluminium inwaters were presented in the literature. In many cases they are based on similarprocedures (Table 1). We have no intention to present all the methods. The aim ofthis review was rather to cover general aspects .of aluminium speciation and tohighlight the most important ideas.

Table 1. Fractionaction of aqueous aluminium

Smnple treatment Aluminium fraction Fraction composition

Acid digestion Total reactive (Alt) Acid soluble fonns

Without digestion Total monomeric Inorganic and organic

(AIm) monomeric complexes

Alt-Alm Colloidal, polymeric and

Acid soluble strong organic complexes

Cation-exchangetreatment

Alulniniunt speciation

Non-labile monomeric

Alm-Alo

Labile monomeric

Monomeric organic

complexes

AI3+, hydroxide, sulfate

fluoride complexes

3

SAMPLING AND STORAGE

In the case ·of speciation analysis, which is in the case of aluIniniurn the oper­ationally defined fractionation· study, sample handling is of great importance sinceany treatment Of natural sample might change tpedistribution of AI. among differentspecies. Stor.age may also affect the chemistry of samples that werenotin equilibriumat the time of collection and when the samples are stored at a.temperaturewhichdiffersgreatly .from their sampling temperatur~.[161.

The experiments, carried out,by ;Fairman elal. [1,7]. haye •• shown "that 'aluminiumconcentratipn at the ,level below 50 •Jlg 1-1 was stablein· high density polyethylenecontainer where 0.1 mol 1-1 nitric .or citric acids were.used~Under tllese conditionslake and tap water samples were stable up to 30 .days. The losses of metal, whichwere later observed, could be ,explained by the precipitation of polymeric hydro­xoaluminium species ratherthan,ad~orption1:>Y the container,material.

" On,e of the most controversial proble,~sconnectedwith aluminiumspeeiationan~lysis is the sample. filtration.. It is; possible .•• that the distribution of aluminiumspecies could change upon filtration._ Many analytical methC).ds require prefilteredsamples (usually through 0.45 JIm filter) togive repeatable results. How,ever" alumi­niurn adsorbed onto~uspendeds,olids (clays, or colloidal particles of humic and fulvicsubstC;lnces) is relatively labile and could account for a considera"ble part of inorganicmonomeric aluminium. fraction measured in \vC;ltet samples [231. Tbe content of thisfraction decreased With, higher suspen~edsoilscont~ntand small filter pore size used.

METHODS OF ALUMINIUM' SPECIATION

The determination of .individual alYllliniumspecies in natural waters is verydifficult due to thdrBarticipation in. dynamic reactions, their low concentration .andt4e presence of complex matrix, mostly of organicorig~n,that might interfere withthe analytical methods applied for the determination. <The. fraction of particularconcern, usually called as, free AI, comprisesAI3+, AIOH2+, AI(OH)i andalu,miniumin very labile complexes (mainly inorgani9).Organically complex~dformsofalumi­nium appear to be muchiess toxic than inorganic forms, particularly. for plants andfish.

Two different types of procedures are used for aluminium speciation. Thetheoretical approach involves the use of thermodynamic data together with analyti-

4 K. Pyrzynska et al.

cally determined content of total aluminium and the significant ligands. The ex­perimental approach involves the analytical separation of various Al species basedon different reaction kinetics with complexing reagent and/or the separation basedon size or charge of determined species.

Theoretical approach

As the sampling of the natural analytes is often associated with a change in theirenvironmental, this can disturbs the existing equilibrium and cause a large uncer­tainty of the obtained results. At the present state of computer science, with availablepowerful computers and data bases of physicochemical constants, it may be a rationalway to make a computational approach. The concentration of different forms.ofaluminium that are present in -a sample could be calculated from the analyticallydetermined content of total aluminium and--the significant ligands followed by thecalculation based on the available equilibrium constants. Some examples ofstich anapproach for Al have been already published [24-28].

Most of the theoretical models describe only inorgani'c monomeric aluminiumspecies without considering polymeric and organic forms. Even when -the authorsdeal with the AI-organic complexes, they choose different ligands, so it is difficultto compare their results [24-26]. In some models the equilibrium reactions with ironand manganese s.pecies are. not included, although aluminium complexation tohydrous particle surfaces has been demonstrated [29,30]. Another problem concernsthe validity and completeness of the thermodynamic data base on which this approachfully depends. Much of the unidentified organic carbon exists as a poorly charac­terised humic material [31]. Discrepancies between analytically measured and cal­culated values arise from model inadequacy as well as analytical errors.

Bertsch and Anderson [28] compared the distribution of aluminium betweenuncomplexed and oxalato and citrato species determined by ion chromatography andcalculated utilising thermodynamic constants. Good agreement was· obtained -- fordifferent ratios of oxalate ions to metal. However, the· citrato complexes wereextremely sensitive to changes in ionic strength and· significant redistribution ofaluminium species was observed. The measured fraction of citrato complexed alumi­nium was lower at the cost of free metal in comparison with the calculated data.

The calculations based on the thermodynamic __ data, working well in modelsystems, fail in water samples because they do not take into account aluminiumassociated with particulate matter. The equilibrium with large organic ligands (humicacids) and aluminium-silicate systems are not know sufficiently well. This as well asthe slow kinetics of several processes may introduce undefined errors. The measure­ments with fluoride ion-selective electrode also did not give concluding resultsb~cause of other ligands competing to aluminium [32].

In conclusion the computation approach may give only fractional information butcannot solve the problem in a more general way. In natural water, often there is noequilibrium, in particular in the case of heterogeneous processes.

Aluminium speciation

Analytical speciation

5

The .experinlental approach in speciation .of aluminium involves· the· separationof. various Al forms based mainly on differential reaction kinetics. with. organiccomplexing .. reagents, non-chromatographic separation of inorganic species fromorganic-bound and colloidal aluminium on solid sorbents and hyphenated chromato­graphic or electrophoretic methods. However, no specific and direct method forseparation and determination of individual aluminium species exists. Only a numberof fractional procedures have been developed to distinguish the groups ofAl species.These procedures usually measure operationally defined metal fractions, which notalways coincident and thus makes. a •direct comparison of the published •methodsdifficult. Although atotal characterisation of aluminium distribution is desired, fromenvironmental point ofview, the most important fraction consists .of the more reactivemonomeric inorganic species. .

Spectrophotometric and fluorimetric reagents. One of the most frequentlyused metpods has been based onreil~ti()nofA13+~nd .its labile~omplexeswith

metallochromic reagents followed by spectrophotolnetric or fluorilnetricanalysis.Short reaction times (2-60s) .~an discriminatebetweellthemonomel"icsp~ciesandthe. Inorestable polymeric complexes..To. minimise' interferences, .mainly fromFe(III), hydroxylamine and 1,10-phenantr()line are used as a masking agent. Thepresence of humic. substances. in natural. waters can. also cause interferences, espe­cially in UV or lower visible region. detection..This problem is usually overcome byextraction of the forlned complex into an organic solvent, which also allows topreconcentrate the analyte and gives a possibility of storing the extract for theanalysis.

8-hydroxyquinoline (oxine) has been often applied to measure the.most quicklyreacting forms of aluminium [245,32-36]. The principle of this method, introducedby Okura et al. [33], is based on the complexation of monomeric Alspecies withoxine at a chosen pH and the rapid extraction of the formed water insoluble complexinto chloroform. The concentration of these species is determined in the organic phasespectrophotoluetrically at 390-420 nm or by flame atomic absorption spectrometry.

Another spectrophotometric reagent that has been used is ferron [37-42]. Thecomplex of aluminium with ferron absorbs light at approximately 370 nm and it is,.as opposite to oxine complex, •fairly soluble in water.. The fraction of aluminium,which reacts almost immediately with ferron, was considered to consist of simplemonomeric species. The second fraction, reacting more slowly, was considered torepresent polynuclear species, while the third one was supposed to represent smallsolid particles. Jardine and Zelazny [40] found that sulphate and fulvate complexesreact with ferron identically to uncomplexed monomeric aluminium. From the otherhand, it was also reported that aluminium complexed with fulvic acid reacts slowlywith ferron [41], while fluorocomplexes react partially for reaction times between15s and 30 min [41]. The ferron method could be a subject to interferences from othermetal ions, especially manganese.

The colour development of aluminium with Pyrocatechol Violet (peV) has beenwidely used for determination of the content of inorganic monomeric Al [5,23,43-

6 K. Pyrzynska et al.

Separator

Extractioncoil (3m)

Pump

mllmin

Buffer

Oxine ----+o-L-:=:-..........---.,...,..

Water---+-"---"-+0--......

47]. An advantage of PCV method compared with earlier mentioned spectrophotome­tric reagents, is that its aluminium complex absorbs light at higher wavelength (580 nm)which makes it less sensitive to interferences from humic substances. Silicon can alsoaffect measurements in silicate-rich water [23]. . '0',

The 8-hydroxyquinoline-5-sulfonic acid(8-HQS) forms watersoluble complexeswith aluminium and has been mostly' used as fluorimetric reagent [48-50]. in thepresence of cationic surfactants (e.g. cetyltrimethylammonium bromide) lO-fold anincrease in the fluorescence intensity was observed [4,8].

The laborious operations of the sample pretreatment and the spectrophotometricmeasurement can be substantially simplified by usingthe methodologies of flo'Y-in­jection analysis (PIA). There are some obvious advantages with this approach, suchas ease of operation, a better control and repeatability of the reaction time, whichmakes it possible to use a very short reaction time, an increased sample throughputand a possibility of mechanisation. Also of very great importance is ateduced humanIJU-J., .....I.'-'.A.IJ ..." ... .I.'-".I..1. in' time-consuming operations such as sample conditio~ing, reagent1'Y\1J.i1""\lr\nl':1ltlr\n and calibration of measuring_system [5'1,52]. Several FIA proce¢ureshave been reported for fractionation of aluminium species using oxine [8,25] orpeV

'[5,41-46] as the' cornplexing reagents. A schematic diagram of the' flow-injectionset-up used by Clarket ai. [81 for aluminium fra~tionationis shown in Figure 1. Afteraddition ofhydroxylamin~and 1,IO-phenantroline for decreasing F~(III)interferen­

ces, a~uminium species react within 2.3 s ~ith oxine at pH 5.0 (acetate buffer). Theformed complex is then extracted at thesame pH value and measured at 390 nm.Thismethod, according to the authors, ,allows to determine the sum of A13+, i.e. itsmonomeric hydroxo species and sulfato, silicato and carbonato complex~s.. Thecomplexes with citrate, humic or fulvic acids and Alp2+ are not included' in~this"quickly reacting" aluminium fraction. '

SampleiD.jecti~n

Chloroform

Figure 1. Schematic diagram' of the FIA system used for the speciation of aluminium (adopted from [8]with kind permission of Elsevier Science-NL

Hawke and Powell' [45] compared the chromophores - pyrocatechol violet,chrome azurol S (CAS) and eriochrome cyanin'R (ECR) for estimatiqn of kineticallylabile fraction, of aluminium using FIA mod~. 1;hei14

; results showed that _PCV re~cts

rather fast. The percentage of aluminium reaGted in lOs were 45% for CAS, 48% forECR and 90% for pev. They observed some changes in Amax for Al-chromophore

Aluminium speciation 7

complexes and the formation ofan isobestic point at A> Amax.It indicated a sequenceof reactions ~ within the first 10 s an intermediate species forms rapidly followed byconversion to the. 1:1 complex. Thus, the authors proposed monitoring absorbance atthe isobesticpoint,to estimate of kinetically labile ( lOs reaction) and equilibriumreactive (10 min reaction) aluminium from a single injection.

Ion exchange methods. From the literature data it follows .that the most widelyused method is that proposed by Driscoll [53]. It is based on the separation ofinorganic monomeric Al from organic complexes'with a cation exchange column.The organic species are predominantly of anionic character and are assumed not tobe adsorbed. In general, the Driscoll method offers a direct measurement of threeoperationally defined fractions:(l) total reactive Al after acid digestion (AIr), (2)total monomeric Al analysed in separate step without digestion and (Altm) and (3)non-labile monomeric AI(Aln1) determined,after passing asample through the columnpacked with cation exchange resin Amberlite IRA-120.,The difference between thetwo last measured values gives inorganic 'monomeric AI. This fraction, consideredto be the more toxic one, is thoughtto include A13+ along withitshydroxo, sulfato,fluoro and silicato complexes. Additiqnally "acid soluble AI" frac;tion can be ob­tained by subtracting total reactive andtotaI'mono111eric aluminium. Driscoll proce­dure is refereed asa classic by now and been often applied with various modification[6,50,54-58]. However, re-equilibrium of the species in a sample can occur duringits passage through the· ion exchange column and organic complexes can releasealuminium. It would result in an overestimation of inorganic monomeric fraction.Observation indicate that the dissociation of organic aluminium, species is oftenvariable and depends on the ratio of aluminium to organic carbon [59,60]. In orderto avoid this the resil1,. has to be conditione~,witha sodium chloride solution, whichmust have the same conductivity as the; water samples for analysis. Thisis to ensurethat the sample pH does not change during analytical procedure. To shorten thispreparative step it is better to condition the resin to achieved its sodium form [17,50].

Chelex 100, the chelating cation exchange resin with iminodiactetate functionalgroups, has a:lsob~en .applied in aluminium speciation schemes to distinguish be­tween resin exchangeabl~(labile) and nonexchangeable aluminium species [61-65].In such procedures the resin has to be very carefully preconditioned with a syntheticsolution containing a mixture of Ca2+, Mg2+~nd ". H,+with th~,col1centratipns similarto those .encountered in natural waters. Some authors suggested that the tendency ofthe resin to shrink makes the batch technique more suitable arid allows a precisecontrol of the contact betwee~ sample and resin [61,62] . However, . in the ba~ch ,method it is not possible to differentiate that it groups the monomeric inorganic andorganic labile complexes together with low molecular weight polYl11er,ic species [61].Chakrabarti tt at. [63.,64] applied Chdex 100 resi.n 3:dditionally to filtrati0rl, ultrafil­tration and anodic stripping voltammetry, to determine the operationally definedvarious aluminium fractions in rivet water, rain and snow. ,

Tqe analytical sch~me for determining .the speciation ~f aluminium in acidicfreshwaters,presented in Fig. 2, included three major operations: filtration, ionexchange and photooxiation[62]:Neither contamination nor analyte loss was ob­served' with 0'.4' J.lin' filters. After UV irradiation the nonexchangeable fraction of

8 K. Pyrzynska et ai.

aluminium, which was not retained· onChelex resin, practically disappeared. Itsuggesting that the major portion of nonexchangeable AI· initially present in thesamples was associated with organic matter. However, in many of the tested samplesappreciable losses of aluminium occurred during photolysis, presumably .due toadsorption onto the walls of the quartz reaction vessels.

Acid-extractableparticulate Al

!~

®

Chelex

aeid digestion

~G

Nonexchangeablefilterable Al

FiltratioD

1

®Totalfilterable

Al

Nonexchangeablefilterable Al

E =B - C Exchangeable filterable fraction

F =C - D Nonexchangeable organicfilterable fraction

Figure 2. Fractionation scheme for speciation of aluminiuln (based on the procedure from [62])

An alternative approach to the speciation of aluminium is based on the selectiveretention of metals (or its rapidly formed complexes) on a suitable adsorbent inamicrocolumn. Such a method after choosing an appropriate reagent and adsorbentcan be very selective and can also involve a preconcentration step. Thus, Fair,man etal.[65]used'thehydrophobic AmberIit(fXA.D~2sorbent packed in very small columnof a volume of 88 JlI to retain a complex formed in an initial 2-3 s time of reactiol,1of aluminium and oxine in a flow-injection manifold. The method was adopted to

Aluminium speciation 9

the manual field.·processing of.samples with subsequent·elution of· the adsorbedcomplex with hydrochloric. acid in the laboratory. The oxine-derivatisedFractogel(vinyl· polymer gel) was also applied for preconcentration of aluminium species[66,67]. The· maximum retention of·aluminium.occursat pH 5.· The· elution can bedone bybackwashing with either 0.05 moll~l He} [66] or 0.02 mol 1-1 of NaOH viatherapidquantitativeconversiQn of resin~bound AI(oxine}Jcomplex to AI(OH)4'[67,68]. The AIF2+ and AIF2+ .species are. retained quantitatively •• and thereforecontribute to the.me~surement·of "free·.Al".The •. peak that follows.injections ofsamples containing organic ligands represents moderately labile Al (Fig. 3b) [67]. Thisis an aluminium fraction which is not sufficiently byretained by the oxine microcolumn.The investigation ofpolymeric aluminium (e.g. Al1304(OH)24(H20 )I!) revealed thatthe Al polymers maybe complexed by the oxinemicrocolumn but not eluted withthe free and very labile fraction ofAl [67] .. This fraction can be quantify usingforitselution 0.2 mol 1-1 NaOH solution and 2 min stopped-flow.

0.6a

Analyte elutionpeak .(Ats;

(]) 0-.4ocas.ca-o 0.2(/J.c«

o

Sampleinjection

IN.ollcolurIu)­reactive AI

Blank elutionpeaks

iIIaHIIII,IIII

"~ .,--_ ,--.. ,_." . . v . tI

IRepeat...

------------~1510

Time, min

5-O.2+------+----__- ..........---......jI--....----I

o

0.8 b Analyte elutionp$ak (AjSn

0.6CDoc:as.0a-oU).0<C.

0.4

0.2

o

Non column­reactive AI

10864

Blank elution..... O.2.....__p_e_a+~s ..... ......__......jI___-.....

o 2

Time, min

Figure 3. Detection of signals for (a)· 5· x 10-3 mol I-I of AP+ in standard solution and (b) in naturalsamples after separation on oxine microcolumn (reprinted from [67] with kind permission ofEls~vierScience - NL)

10 K. Pyrzynska et aL

Chromatographic and electrophoretic methods. High-performance 'liquidchromatography (HPLC) is also a very useful technique in speciation analysis ofaluminium. Because of the ability to determine of individual species, this methodmay provide the most probably information on aluminium species in aqueous systems[28,48,49,69-71]. It can also separate fluoride species from the other inorganicmonomeric Al fraction, which is difficult with other described methods. However,several problems occurred, including the inability to quantitatively detection ofstronger aluminium organic complexes at the.low concentration in surfacewater[69],interferences from some metal ions including iron [48,69], poor chromatographicseparation of species using isocratic elution [46,47] and the dissociation of alumi­nium complexes on the cation exchange column when pH and ionic strength of theeluent and sample are not identical, which resulted ina poor agreement with thethermodynamic calculations [28]. Also,a high concentration of organic matter couldaffect the columns and reduce their useful life time.

An analysis of lake water samples (pH 5.31) by HPLC indicatf;d three peaks [70].The first peak corresponds to the species with a charge less than or equal to +1 andis assigned to organically bound aluminium. The species with a charge of +2correspond to the second peak and represent Alp2+ and possibly other Al organiccomple~es. Finally, the third peak represents A13+ and its soluble hydrolysis species.The measurement of this sample indicated that a considerable amount of the totalmonomeric alun1inium (65-70%) is organically bound.

An ion-pair chromatographic method was also evaluated for speciation of alumi­nium at concentrations down to a, fe~ g JlI- I [70]. The stationary ph,ase of the columnconsisted of a neutral,ul1polar polystyrene-divilylbenzene resin ~ith a large surfacearea. Butane sulfonic acid Was used as: lipophilic ion added to the mobile phase(NH4CI, pH=3.0). It was found that alu,minium fluoride and several five- or six-mem­bered complexes (oxalato, citrato, tartrato, malonato) are sufficiently stable to bedetermined. But their stability depends on pH of both the sample and the eluent andon residence time on the chromatographic column.

Usually in HPLC aluminium determination fluorimetric detection has been donewith a post column reagent such as 8-HQS [48,49], pev [69] and lumogallion [70].The lowest reported detection limits were 0.95 ~g 1-1 [48] andO.19 ~g 1-1 [70].

The reverse-phase chromatography was also,used for speciation of aJuminium inthe presence of humic acids [71]. The best separation was achieved on,RP-18HCDcolumn, when acetonitrile-water (85-1?% v/v) solution was used as an eluent.Determination of aluminium in 0.2 ml fractions was performed by off-line GFAAS.The results shows clearly (Fig. 4) that a part of the metal is eluted before the humicacid peaks appear on the chromatogram. The biggest part is hoWever eluted togetherwith some components of humic acids dissolved in the sample.

Capillary zone~ electrophoresis (CZE)"was applied fot analysis of fluoro- andoxalatoaluminium complexes present in aqueous solutions [72]. Detection was per­formed using indirect absorption method with a background electrolyte containingimidazole. The detection limit for uncqrnplexed Al and complexed fluor9al~minill~

species was found to be about 10 nmol I-I. The, CZEmethod possessesa,distinctadvantage over HPLC for the separation of sample solutions containingfluoro and

Aluminium speciation 11

oxalatocomplexes. Using HPLC only uncomplexed Al species and AIF2+ produceddiscrete chromatographic peaks while AIF!· and monooxalatoaluminium speciesco-eluted as a single peak under isocratic conditions [28].

-= 0.07 0.35 -=0~ 0.06 0.30 .Su "!-l(I,) U... QjQj

0.05 ~~

en "'t:S~ 0,04 0.20 en

~ <0.03 0.15 ~2- ~(I,) 0.02 0.10c.J Qj= u

Cd 0,01 0.05 I::"Q t't:II-i ..00

0.00 J-cen 0.00 0..c V.1< ,.Q-0,01 -0.05 <co lO 0 lO 0

0 0 ,...l v-4 e-.i

Time,·min

Figure 4e Chromatogram profile of humic acid with superimposed aluminium elution profiles determinedby GFAAS (represented by bars). Conditions: RP-18HCD column, isocratic elution \Vithacetoni­trile-water (85-15% v\v) solution of 1 ml min-1 flow rate, fraction volume 0.2 ml [71]

VALIDATION OF METHODS FOR ALUMINIUM SPECIATION STUDY

Operational speciation, to ",hich belongs the methods of aluminium~peciationis generally difficult to be properly validated. The developed procedure are based. onthe relative inertness/lability of the species and result in a strong infJuence ofnumerous experimental· parameters, such as concentration of reagents used, thepresence and concentration of accompanying species, .time, temperature or mixingconditions of reaction involved. Those parameters. are usually difficult to be stan­dardised. Difficult to standardise is also kinetics of many reactions of aluminium insolution, especially in the samples containing poorly defined organic matter. On theother hand it is known, that some. reactions, as e.g. the· monomeric - polymericexchange and the reaction involving pH changes,may be relatively rapid in labora­tory and· also in environment.

This,. at least partially, explains the lack of reliable certified reference materialfor the study of aluminium speciation. The complexity of the system also restricts apossibility of application of the recovery tests for validation of the method. Thereforesuch validation must 'be based on the comparison of different procedures, and acareful interpretation of all steps of the procedures and final results· must be made.Such a comparison has been carried out and, as a standard procedure the Driscollmethod with pyrocatechol violet detection of the aluminium fractions was accepted.Those fractions correspond to (a) acid reactive aluminium (AIr), (b) tot&l monomericaluminium (Altm) and (c) non-labile monomeric aluminium (Alnl).The dependenceon experimental condition is indicated by the necessity of keeping the exact times ofreactions. In this respect the use of FIA is of advantage, because of better reproduci­bility of conditions and in consequence of results.

12 K. Pyrzynska et al.

In order to clarify the measurement of the aqueous aluminium species theprogram named "The Standards. Measurements and Testing Programme" (formerlyBCR) initiated the interlaboratory study for the determination of "labile monomericAI" fraction [17]. The distributed samples originated from lake and tap waters andafter spiking with AI3+, the final aluminium concentrations ranged from 25 to 1000Ilg 1-1 (pH =5.2-7.0). The Driscoll method with pyrocatechol violet detection [50]was used as a reference method by each laboratory and also an alternative methoddeveloped in-house by each participant. Among them was the FIA system with thefluorimetric detection using 8-HQS [50], HPLC method with a post-column derivati­sation follow~d by the fluorescence detection [40] and the flow-injection Driscoll­PCV method [73]. Generally, a good agreem.ent between the reference andalternative methods was obtained for the samples with small additions of aluminium[17]. The content of labile monomeric Al in test samples without any additional Alspike was very close to the detection limit for frequently used Driscoll method. Thesamples \vith the highest aluminium additions gave few useful data due to theprecipitation of aluminium. When evaluating these results, one must keep in mindthat speciation measurements in samples with such low levels of monomeric alumi­niurn fraction is very difficult.

From the practical points of view, it is most important to have a reliable procedurefor determination of toxic species, which are the AIOH2+ ions. However, there is nodefinite proof that other species such as AI(OH)!, Al02" or even AICI2+ whichrelatively rapidly change into AIOH2+ should not be considered similarly as toxic. Inconsequence two questions should be answered: (1) which species should bedefi­nitely determined as toxic, and (2) how thedetetmination of operationally definedspecies can be validated [74].

CONCLUSIONS

The environmental effect of aluminium depends on the form in which it occursin natural waters. The direct measurement of specific forms in real samples is verydifficult. Thus a number of procedures have been developed to distinguish betweenbroad groups of aluminium species. A proposed procedures usually is based onmeasuring Qperationally defined aluminium fractions, which are not alwayscoinci­dent, and makes a direct comparison between the published results very difficult.

Several authors of the papers concerning alulIlinium speciation tried to obtain theresults for as many as possible aluminium fractions. However, in a many of cases thecomplete knowledge of the distribution of Al is not needed. There seems to be ageneral agreement that the most reactive and toxic aluminium forms are A13

+,

AIOH2+, AI(OH)! and Al in very labile complexes (mainly inorganic). These speciesare included in the inorganic monomeric fraction determined spectrophotometricallywith a short reaction time. They are also measured in exchangeable inorganic AI'group. Therefore, for the evaluation of the chemical behaviour of aluminium innatural aquatic systelTIS, colloidal Al must be given a consideration because it canrepresent an essential fraction of this metal in freshwater [16].

Aluminium speciation

Acknowledgement

This work has been partially supported by the Tubitak-Doprog (Turkey) research grant.

REFERENCES

13

1. Clarke N., Danielsson L.G. and Span~nA., Pure & Appl. Chem., 68,1597 (1996).2. Lever A.B., Coord. Chem. Rev., 149,1 (1996).3. Stumman W. and Morgan J.I, Aquatic Chemistry, 2nd,edn, Wiley, New York 1981, p.154.4. Bowen HJ., Trace Elements in Biochemistry, Academic Press, New York 1966, p.67.5. Quintela M.I, Gallego :tv1.and Valcarcel M., Analyst, 118, 1199 (1993).6. Canizares P. and Luque deCastro M.D., Anal. Chim. Acta, 259, 59 (1994).7. Zernichow L.. and Lund W., Anal. Chim. Acta, 300, 167 (1995).8. Clarke N. and Danielsson L.G.,Anal. Chim.Acta, 306, 5(1995).9.AnderssonM., Water Air Soil Pollut., 39, 439(1988).

10. Wren C.D. and Stephenson G.L., Environ. Pollut., 71, 205(1991).11. Wr6bel K, Blanco Gonzalez E. and Sanz-Medel A., Trace Elements in Medicine, 10, 97 (1993).12. King S.W., Savory J.andWills M.R., Crit.Rev. Clin. Lab. Sci., 14, 1 (1981).13. Sanz-MedeIA., Fairman B. andWr6bel K., Element Speciation in Bioinorganic Chemistry., [Caroli S.,

Ed.) Wiley & Sons, New York, 1996.14. Burgess J., Analyst,!17, 605 (1992).15.MartellA.E.,Motekaitis R.J. and SmithR.M., Polyhedron, 9, i 71 (1990).16. Lydersen E., Salbu B. and Poleo A.B.S., Analyst, 117,613 (1992).17. Fairman B., Sanz-Medel A., G~111ego M., Quintela P., Jones P. and Benson R.,Anal. Chim.Acta, 286,

401 (1994).18. Forbes W.F., HaywardL..M. and AgwaniN., Lancet, 338,1592(1991).19.5mithR.W., Coord. Chem.Rev.,149, 81 (1996).20. Birchall J.D., Bellia J.P. and Roberts N.B., Coord. Chem. Rev., 149,231 (1996).21. Laksman S., MillsR., Patterson H. andCronanC., Anal.. Chim. Acta,.282, 101 (1993).22.Plankey BJ., Patterson H.and Cronan C., Anal. Chim. Acta, 300, 227 (1995).23. Goenaga X., BryantR. and Williams D.J.A.,Anal. Chem., 59,2673 (1987).24. LewisT.E.,Dobb D.E., Henshaw I.M., Simon S.J. and Heithmar E.M., Int. J. Environ. Anal.. Chem., 34,

69 (1988).25. Clarke N., Danielsson L.G. and Sparen,lnt. J. Envviron. Anal.Chem., 48, 77 (1992).26. FurreG., Trusch B.and Miller C., Cosmichim. Acta, 56,3831 (1992).27. Shuping B., Analyst, 120,2033 (1995). .28. Bertsch P.M. and Anderson M.A., Anal. Chem., 61, 535(1985).29. Charlet L., SchindlerP.W., SpadiniL., FurrerG. andZysset M., Aquati. Sci., 55,291 (1993).30. Ohman L.O. and Sjoberg S., Coord. Chem. Rev., 149, 33 (1996).31.JQhson K.S., Coale.K.H. and Jannasek H.W., Anal. Chem., 64, 1065A (1992).32. Lazerte B.D., J. Can. Fish AquaticSci., 43, 776 (1984).33. Okura T., Goto K.and Yotuyanagi T.,Anal. Chem., 34,581 (1962).34. Barnes R.B., Chem.Geol., 15,177 (1975).35. Lalande H. and Hendershot W.H., J. Can. Fish Aquatic Sci., 43,321 (1986).36. Danielsson L.G., Sparen A. and Glynn A.W., Analyst, 120,713 (1995).37. GotoK., Tamura H., Onodera O. and Nagayama N., Talanta, 21, 183 (1974).38. Buffle I, ParthasarathyN. and Haerdi W.,Water Research, 19, 7 (1985).39. Parker D.R., Kinraide T.B. and Zelazny W., SoilSci. Soc. Am. J., 53, 789 (1989).40. Jardine P.M. and Zelazny W., Soil Sci. Soc. Am. J., 51,889 (1987).41. Hodges S.C., Soil Sci. Soc. Am. J., 51, 57 (1987).42. Alva A.K., Sumner M.E., Li Y.C. and Miller W.P., Soil Sci. Soc. Am. J., 53, 38 (1989).43. ExleyC. and Birchall J.D., Polyhedron, 11, 1901 (1992).44. Morrison G.M., Analyst, 115, 1371 (1990).45. Hawke D.J~ and Powell H.K.J., Anal. Chim. Acta, 299, 257 (1994).46. Kramer J.R., GleedJ. and Gracey K.,Anal. Chim. Acta, 284,599 (1994).47. Riseng C.M., Gensemer R.C., ShosaJ.D. and Driscoll C.T., Soil Sci. Soc. Am. J., 56, 449 (1992).

14 K. Pyrzyfiska et a1.

48. Garcia Alonso J .J., Lopez Garcia A., Sanz-Medel A., Blanco Gonzales E., Ebdon L. and Jones P., Anal.Chim. Acta, 225,339(1989).

49. Jones P., Int. J.Environ. Anal. Chem., 44,1 (1991).50. Fairman B. and Sanz-Medel A., Int. J. Environ. Anal. Chern., 50, 161 (1993).51. Luque de Castro M.D., Talanta, 33,45 (1986).52. Campanella L., Trojanowicz M. and Pyrzynska K., Talanta, 43, 825 (1996).53. Discoll C.T., Geological Aspects of Acid Deposition, [Bricker 0., Ed.] in Acid Precipitation, Butter-

worte, London 1989.54. Goenaga X and WilliamsDJ.A., Environ. Pollut., 14, 1131(1988).55. Christophersen N., Neal C., Vogt R., Esser J.M. and Anderesen S., Sci. Total Environ., 96, 1175 (1990).56. Alvarez E., PerezA. and Calvo R., Sci. Total Environ., 133, 17 (1993). .57. Bull K.R. and Hall l.R., Environ. Pollut., 12, 165 (1988).58. Achilli M., Ciceri G., Ferrari R., Culivicchi G. and Pieri S., WattirAirSoil Pollut., 57, 139 (1991). '59. Backes C.A. and Tipping E., Int. 1. Environ. Anal. Chem., 30, L35 (1987).60. Berggren D., Int. 1. Environ. Anal. Chem., 35, 1 (1989).61. Miller J.R. and Adelaman lB., Water Research, 21, 999 (1987).62. Campbell P.G.C., Bisson M., Bougie R., Tessier A. and Villeneuve J.P., Anal. Chem., 55, 2246 (1993).63. Chakrabarti C.L., Lu Y., Cheng 1., Back M.H. and Schroeder W.H., Anal. Chim. Acta, 267,47 (1993).64. LuY., Chakrabarti C.L., Back M.H., Gregoire D.C. and Schroeder W.H., Anal. Chim. Acta, 293, 95

(1994).65. Faitman B., Sanz-Medel A. and Jones P., J. Anal. At. Spectrom., 10, 161 (1995).66. Resing J.A. and Mottl M.l, Anal. Chem., 66,4105 (1994).67. Simpson S.L., Powell KJ. and Nilsson N.H.S., Anal. Chim. Acta, 343,19 (1997).68. Downard A.J., Lenihan R.J., Simpson S.L., O'Sullivan B. and Powell KJ., Anal. Chim. Acta, 345,5

(1997).69. Willett LR., Soil Sci. Soc. Am. 1.,.53, 1385 (1989)~70. Suitheimar S.H. and Cabaniss S.E., Anal. Chern., 67, 2342(1995).71. Wojciechowski M., MSc Thesis, University of Warsaw, Department of Chemistry (1997).72. Wu N., Horvath WJ., Sun P. and Huine C.W., J. Chromatogr., 635, 307 (1993).73. LaZerte B.D., Chun C., Evans D. and Tomassini F., Environ. Sci. Technol., 22, 1106 (1988).74. Fairnam B., Sanz-Medel A., in Quality assurance in Environmental Analysis [Quevauviller P., Maier

E.A., Griepink B., Eds], Elsevier, Amsterdam 1995, pp.215-233.

Received May 1998Accepted July 1998